Currently it is known to use, for the application of adhesives to substrates made of various materials, such as for example fabrics or ribbons made of plastic material, spreading devices which comprise one or more spreading heads provided with a duct for feeding the adhesive, the delivery end of which faces, during use, a substrate which is made to advance in close contact therewith in the form of a ribbon.
The adhesive is introduced in the duct in the liquid state, typically by means of appropriately provided gear pumps, and can be applied either continuously or intermittently, by throttling the flow thereof by means of appropriately provided valves of a known type which are arranged in the spreading head.
Such valves further allow to vary the width of the region of application of the adhesive and to perform throttlings of the adhesive, with a preset pitch, transversely to the ribbon of substrate.
As an alternative to the valves, the variation of the width of the region of application of the adhesive can also be achieved by means of one or more inserts of a known type, which can be arranged automatically or manually within the duct so as to partially obstruct its cross-section, so as to adapt its width to the width of the ribbon.
It is also known to insert, transversely to the duct, appropriately provided contoured laminas, which are adapted to obstruct the duct partially so as to be able to obtain a selected distribution of the adhesive transversely to the substrate; it is thus possible to achieve, for example, a distribution of the adhesive which affects uniformly the entire substrate or also a distribution of the so-called “multiline” type, which is constituted by a plurality of longitudinal layers of adhesive which are mutually parallel and spaced.
These known types of spreading head, however, have drawbacks: first of all, they do not allow to obtain layers of different adhesives arranged side-by-side or laterally adjacent layers of a same adhesive with different grammages.
Moreover, the use of these known types of head entails a waste of adhesive if an adhesive of higher value and/or grammage (therefore also having a structural function) is required only in some regions of the substrate while in other regions an adhesive of lower value and/or grammage is sufficient, since it forces to use the higher-value and/or higher-grammage adhesive for all the regions of the substrate.
Moreover, said known types of head allow to obtain only a uniform film of adhesive in contact with both surfaces of the materials to be coupled; this entails the need to use a high-value adhesive even if its use is required only by one of the two surfaces to be coupled, with an additional waste of material.
Further, if the substrate is porous, the use of known types of head entails an additional waste of high-value adhesive, since said adhesive also acts as a filler for the pores of the substrate.
There are also applications in which such known types of head are used to provide membranes which in some regions must have breathability characteristics, and therefore require low grammages of applied adhesive, and in other regions must instead provide a vapor barrier effect and therefore require high adhesive grammages; to achieve this embodiment, known types of head require a double passage over the substrate, which is performed either with different heads or with a same head which is modified at a later time, and this increases the production times, and therefore the production costs, of producing said membranes.
Moreover, since many thermoplastic adhesives have a residual stickiness after their spreading, it is not possible to apply them with a double pass.
In the case of cold adhesives, spreading heads are known which allow to apply multiple superimposed layers; however, such heads do not allow to perform combined applications, i.e., multilayer products in certain regions and single-layer products in other regions, and also do not allow intermittent and/or combined applications of the so-called “multiline” type.